1. Field of the Invention
The present invention relates to a polymeric gel electrolyte and to a lithium battery employing the polymeric gel electrolyte. More particularly, the invention relates to a polymeric gel electrolyte having a good capability of impregnating an electrolytic solution, having good mechanical properties and ionic conductivity characteristics, and having improved adhesion to electrodes. The lithium battery of the invention that utilizes the polymeric gel electrolyte has improved charging/discharging characteristics and efficiency, and has long lasting life and shelf characteristics.
2. Description of the Related Art
Secondary batteries have increasingly become essential components of various portable electronic devices and telecommunications equipment, such as portable audio devices, cellular phones, camcorders, notebook type computers, and the like. The main bodies of such devices and equipment are becoming smaller and smaller, and the size of the secondary batteries has an impact on how small these devices can become. One factor to consider in the manufacture and sales of portable telecommunications equipment is the possibility of long lasting use of such devices and equipment. In particular, lithium polymer batteries are thin and light, just like paper, and have considerable flexibility with respect to their shape. Lithium polymer batteries employing polymer electrolyte also tend to be free from danger of leakage or explosion, and exhibit improved safety, unlike lithium ion batteries using liquid electrolytes. However, the lithium polymer batteries have a lower discharge capacity, and the manufacturing process thereof is complicated, when compared to the lithium ion batteries. In addition, it is expensive to manufacture these lithium polymer batteries.
Artisans today therefore have conducted various types of research into making and using polymer electrolytes, mostly the gel type, that have desirable conductivity characteristics at room temperature. The polymeric gel electrolytes usually are prepared by adding a large amount of liquid electrolyte into a polymer matrix. These polymeric gel electrolytes are known to substantially contribute to the practicability of lithium polymer batteries.
In the above-described gel polymeric electrolytes, the polymer matrixforming polymers that typically are used include polyacrylonitrile, polyvinylidenefluoride, polyethyleneoxide, polymethylmethacrylate and polyvinylchloride. These polymeric gel electrolytes have encountered, however, the following disadvantages when used in lithium polymer batteries.
Adding a large amount of an organic electrolytic solution to a polymer matrix may provide polymeric electrolytes having poor physical properties, (e.g., causing internal shorting). Large amounts of organic electrolyte solutions also may increase the thickness of the resulting film used to form the battery, sharply deteriorating the battery performance during charging/discharging, which also may deteriorate the battery performance at a high current rate. In addition, since the organic electrolytic solution typically is highly volatile, it is difficult to accurately adjust the content of the electrolytic solution during the course of preparing the polymeric gel electrolyte. The foregoing disadvantages are even more problematic when the electrolytic solution is not uniformly distributed in the battery or the content of the electrolytic solution is not adjusted accurately, because nonuniformity in current flow may occur during charging/discharging of the battery, thereby resulting in deterioration in battery performance.
To overcome the foregoing disadvantages, i.e., improve physical properties of polymeric electrolytes, the art has proposed a method in which a porous film is used as a support body of a polymeric gel electrolyte. For example, U.S. Pat. No. 5,681,357, the disclosure of which is incorporated by reference herein in its entirety, discloses a lithium secondary battery prepared by forming a cell by coating a porous polyethylene film with a polyvinylidenefluoride solution and drying, and then injecting an electrolytic solution into the resulting structure and gelling the same at high temperature. Japanese Laid-open Patent Publication Hei 10-162802 discloses a separator prepared by coating or impregnating a polymeric gel electrolyte such as polyacrylonitrile into a porous insulating film.
Support bodies of a polymeric gel electrolyte that are comprised of a porous film typically are prepared by gelling the polymer that is coated on the porous film at high temperatures. This process is carried out primarily due to the poor compatibility between the polymer and the electrolytic solution, and because it improves the workability of the polymeric gel electrolyte. There still are safety problems due to leakage of the electrolytic solution. Further, since most polymers that are coated on the porous film are hydrophilic, they can be easily peeled off from the porous film, which usually is made of hydrophobic materials.
Japanese Laid-open Patent Publication Hei 11-313535 discloses a process of preparing lithium secondary batteries using polymeric gel electrolytes. The polymeric gel electrolytes are prepared by mixing polyvinyl idenefluoride with an electrolytic solution comprised of a lithium salt and an organic solvent to produce a mixture, coating an electrode with the mixture, and then heating. The lithium secondary batteries prepared in accordance with this process exhibit improved adhesion between the electrodes and the polymeric gel electrolyte, and they have a more uniform distribution of the electrolytic solution. If the polymeric gel electrolyte composition containing an organic electrolytic solution is previously coated on the electrode, however, the solvent contained in the composition may be volatile. Thus, an organic solvent having a high boiling point must be used as the organic solvent. In addition, low humidity conditions typically are required during the process of making the lithium secondary batteries.
Carbon, and in particular graphite, which usually is prepared through high-temperature annealing, generally is used the anode active material in lithium ion batteries or lithium polymer batteries. When using this anode active material, the charging/discharging potential curve sharply varies depending on the type of organic solvent employed in the electrolytic solution, thereby resulting in a great variation in the irreversible capacity and battery efficiency. In particular, employing propylene carbonate (a high boiling point solvent) as the organic solvent for the electrolyte solution used to prepare lithium ion batteries and lithium polymer batteries, produces a large irreversible capacity of the anode, and causes severe side reactions, such as gas generation. Thus, only a minimum amount of propylene carbonate typically is used or its use is limited. Accordingly, there is a crucial demand for development of high boiling point organic solvents that can replace propylene carbonate, or for developing new anode materials for use in lithium batteries that employ polymeric gel electrolytes.
The description herein of certain advantages and disadvantages encountered by previous systems is not intended to limit the present invention. Indeed, the present invention may include some or all of the features of previous systems without suffering from the same or similar disadvantages.